1. Field of the Invention
The present invention relates generally to the art of telecommunication and the multiplexing of digital information. More specifically, the present invention relates to a SONET multiplexed system having the capability of interfacing digital communication lines operating at different data rates.
2. The Background Art
The SONET (Synchronous Optical NETwork) communication hierarchy has been specified by the American National Standards Institute (ANSI) as a standard for a high-speed digital hierarchy for optical fiber. The SONET standard is described in detail in ANSI T1.105 and T1.106, and in Bellcore Technical Recommendations TR-TSY-000253, which are incorporated by reference into this application.
The international realization of the SONET communication hierarchy is the Synchronous Digital Hierarchy (SDH) standard promulgated by the ITU-T (the successor organization to the CCITT) via Recommendations G.707-709. The salient difference between SONET and SDH is that the basic SDH rate, known as Synchronous Transport Module level 1 (STM-1), is 155.52 Mbps and the basic SONET rate, known as Optical Carrier level 1 (OC-1), is 51.84 Mbps. The STM-1 rate is equivalent to the rate of Optical Carrier level 3 (OC-3) of SONET. The respective rates of transport according to the SONET and SDH standards are shown in TABLE 1.
TABLE 1 ______________________________________ SONET Optical Carrier and SDH Synchronous Transport Module Levels Line Rate (Mbps) SONET Level SDH Level ______________________________________ 51.840 OC-1 155.520 OC-3 STM-1 466.560 OC-9 622.080 OC-12 STM-4 933.120 OC-18 1244.160 OC-24 STM-8 1866.240 OC-36 STM-12 2488.320 OC-48 STM-16 ______________________________________
Previously there have been disclosed devices for interfacing digital communication lines operating at different rates. The patents to Huizinga et al. (U.S. Pat. No. 4,416,007) and Wurst (U.S. Pat. No. 4,606,021) are directed to "conference call" mechanisms where digitized voice data is summed with other similar signals, and the resulting signal is placed in an appropriate, selected time slot on a Pulse Code Modulated (PCM) highway which operates at a fixed data transfer rate. The patents to Hartmann et al. (U.S. Pat. No. 4,510,596) and Urui et al. (U.S. Pat. No. 4,636,253) are directed to methods of supporting rapid time slot re-assignment in PBX systems that internally carry voice and data services over a PCM highway operating at a fixed data transfer rate. These four patents have in common various interface devices or data ports which support various services, but all eventually interface to the PCM highway at a fixed data transfer rate.
The patent to Narasimhan (U.S. Pat. No. 4,771,418) is directed to an integrated circuit which supports the basic rate integrated services digital network (ISDN) S-type subscriber interface into a Private Automatic Branch Exchange (PABX).
Conventional interfaces for PCM highways as exemplified by the above-referenced patents suffer from being expensive and unreliable, and are therefore disadvantageous. A conventional approach is to develop a different integrated circuit to handle each required interface configuration. Accordingly, the cost for development of conventional interface systems, and the cost of maintaining them is high. Each time a new interface configuration is required, a new circuit must be developed to implement the new interface. Each different interface circuit developed requires its own distinct parts inventory and troubleshooting and maintenance processes.
Conventional interfaces which use a time shared bussing architecture are also disadvantageous in that they must operate the tributary units at a fixed data rate equal to the highest data rate of the PCM highway. Operating at such a high data rate uses a great deal of power. Higher power consumption means higher heat generated, necessitating extreme ventilation measures. This too is also a problem.